The present invention relates to electrical power distribution systems. More specifically, the present invention relates to a method and apparatus for injecting harmonic currents into constant frequency and variable frequency distribution systems.
Many aircraft include power distribution systems for supplying ac power to loads such as ac-to-dc converters, ac-to-ac converters, on-board electronics, and electromechanical/electrohydrostatic actuators. Many of these loads contain non-linear devices such as bridge rectifiers and inverters.
The non-linear devices can create harmonic currents on the system""s power distribution line. The harmonic currents can disrupt other loads connected to system""s main power supply (e.g., electrical generators, inverters) and cause malfunction, and even failure, of the other loads.
It would be desirable to remove the harmonic currents without cutting into the power distribution line and measuring main distribution current. In aircraft power distribution systems, the main distribution current can be quite large. For example, a 115/208 volt, three-phase 150 KVA generator has a rated current of approximately 450 amperes.
According to one aspect of the present invention, a power distribution system includes an ac power source; a power bus connected to the ac power source; a capacitor bank shunt-connected to the power bus; and an active filter shunt-connected to the power bus. The active filter includes current sensors, an inverter and an inverter control. Each current sensor senses current flowing through a corresponding capacitor of the capacitor bank. The inverter control, in response to the current sensors, controls the inverter to inject harmonic currents into the power bus, thus supplying the harmonic current demands of the non linear loads.